THE RECENT PROGRESS OF SOLAR PHYSICS. 7 



versy shows us to how limited a company of si^ecialists we must look 

 as judges in matters so important. 



The instrumental aids of our study have grown in the period un- 

 der review with the demand for greater accuracy, until the detached 

 prisms of Kirchhoff's apparatus are replaced by trains of automatically 

 adjustable mechanism, giving us in Thollon's recent instrument the 

 equivalent dispersion of thirty prisms of fliiit, or what has replaced 

 the " gitter " of Fraunhof er, that wonderful product of skill, the Ruth- 

 erf urd grating, which for a large variety of uses has already supplanted 

 the prism. Observatories especially devoted to solar physics are being 

 established by European governments, as at Potsdam by Prussia, and 

 at Meudon by France. I have already alluded to that on Etna, and I 

 hope it will not be long before we have a distinctly physical observa- 

 tory within our own territory. There is no step in our power to take 

 which promises so much for immediate advance as the installation of 

 one in a suitably elevated station, for certain investigations can be 

 made only under this condition, and no amount of instrumental appli- 

 ance, patience, or skill, at a lower altitude, supplies their place. 



In now reviewing the acquisitions which this twenty years' labor 

 has brought us, we can not but agree that we have achieved a great 

 deal, and yet must admit, with wonder at the field still before us, how 

 little is our progress in comparison w^ith what remains unknown. 



"VVe have found out how to detect daily the outbursts from the sun 

 which were before invisible, but we watch these outpourings of enor- 

 mous forms without yet knowing what drives them forth, without be- 

 ing sure how far our very view is not in part illusion. 



We have learned how to study and fix many of the wonderful de- 

 tails of spot-actions without knowing what spots are. We see them 

 presenting themselves in increasing importance through a term of 

 years, and then diminishing, and we attempt to assign a period to 

 these cycles of gi-owth and decay. This period is often fixed at about 

 eleven years, with a perhaps unjustifiable confidence, for we can not 

 be said to know whether what we have seen in so brief a time is con- 

 stant or variable, nor whether it be not the mere incident of some 

 greater cycle, whose course began before man was here to see it, and 

 whose term may not be complete till he has gone. 



We are possibly now led to ask what our science has taught us on 

 the connection of these remote changes with questions which affect 

 our daily lives, and perhaps to put the utilitarian question, " What is 

 all this worth ? " 



We find at the present time our study growing into a closer union, 

 not merely with stellar astronomy on the one hand and terrestrial me- 

 teorology on the other, but with all the physical sciences, than would 

 once have been supposed possible. Thus, to give a single instance, 

 whatever be the result of the discussions aroused by Mr. Lockyer's 

 statements, it seems likely that we are to look to the analysis of the 



